1. Field of the Invention
The present invention relates to a method of reducing multipath propagation effects when processing detection mode “S” replies from targets such as aircraft, by secondary surveillance radars (commonly called SSR).
2. Description of Related Art
In some radar signal echo reception cases, this reception can be polluted by spurious signals due to strong multipaths in different directions. In these conditions, the known current mode S signal processing operations cannot correctly process the mode S replies. This results in unacceptable aircraft detection losses.
It should be recalled here that the principle of the mode S is the selective interrogation of aircraft by exploiting the monopulse information in order, in a single interrogation in the lobe, to “locate” and “decode” with virtual certainty (calculation of a CRC, that is, a cyclic redundancy code) the message transmitted by the onboard transponder. Algorithms have therefore been developed to this end that therefore allocate all the aim to the processing of a reply.
The characteristics of the Mode S standard (ICAO standard, Annexe 10) are:                The main objective is to perform the detection and location (in 3D: azimuth, distance, altitude) of an aircraft in a single interrogation. Thus, EUROCONTROL, just like the STNA in France, have defined a metric: the number of interrogations per radar antenna revolution and per aircraft, in addition to the probability of detection. It can thus be seen that, in addition to the conventional radar performance, the manner in which it is obtained is deemed important (efficiency indicator),        The mode S reply (see a simplified example in FIG. 1) is, by construction, much longer (64 μs or 120 μs) and denser than the SSR reply (21 μs) and consequently more sensitive to the multipaths: the space between two mode S pulses is either 500 ns or 1 μs, whereas it is of the order of 1 μs, 2.45 μs, 3.45 μs, . . . for an SSR reply. Therefore, the probability that a multipath of a reply will pollute the pulses of that same reply is much higher in mode S than in SSR.        The data exchanged between the ground and the onboard system must be reliable: an error rate of 10−7 is required by the mode S station specification according to the EUROCONTROL standard. For this, the standard has provided an error correcting code (24-bit CRC) that makes it possible to detect whether the message is corrupted. This code has been designed as a reply to the garbling between conventional secondary replies (21 μs in duration, no more than fourteen 450 ns pulses polluted, or on average eight pulses polluted per reply, distributed over 21 μs).        In practice, to meet the exchanged data security requirements, the correction is carried out on at most 10 bits spaced apart by less than 24 μs in the mode S message. Thus, an SSR reply that is richer in pulses than the average (code having more than six pulses out of the possible twelve), can pollute more than 10 bits of the mode S message, and consequently render a mode S reply uncorrectable (see FIG. 2).        The signal processing handles both the decoding of the mode S reply and the marking of the bits of the message (1 bit lasts 1 μs) that may be errored (poor quality). According to the principle of the mode S standard, it is only these marked bits that can be used for the correction by the error correcting code.        
This concept, implemented in a particular propagation environment, faced with “garblings” (spurious signals that evolve from interrogation to interrogation because of the selectivity of the interrogation, is fully operational. In the presence of strong multipaths, which remain by definition “glued” to the reply, the concept no longer stands up: each reply is analyzed and rejected independently of the other.
In practice, with the currently known methods, the mode S replies received are systematically self-polluted for each multipath:                by the “on line” reflections (in the axis of the antenna)        and/or “laterally” (slightly offset from the axis of the antenna).        
The processing of the mode S signal is optimized for the processing of a reply for each lobe, and therefore the decoding and the correction of the message are performed on a single reply. When there is a failure, a new interrogation is automatically sent and once again the signal processing function (hereinafter simply called TS) exploits the new reply. In the presence of multipaths, the failure is repeated. As long as the target is in the receiving lobe of the radar and the reply has not been able to be decoded, a new interrogation is generated. Therefore, when the multipaths are strong, the number of selective interrogations for a polluted target can, ultimately, be equal to that which would be produced by the secondary processing in non-selective mode. However, since the decoding decision it taken on each reply, there is global failure throughout the lobe.
The devices of the prior art were focused mainly on the signal processing (TS) to best perform the decoding and quality assignment functions since then, through the mode S standard, the error detection method and the effectiveness of the correction were imposed by the code employed and the false corrections rate required.
For each reply, the TS exploits the information available at the output of the receiver, which is linked to the radar antenna, namely:                the power detections on the SUM and DIFFERENCE pathways,        the phase information representing the misalignment of the target in the lobe (information called “monopulse” and referred to as such hereinafter).        
With reference to FIGS. 3 to 5, there now follows a description of three typical cases of pollution of the mode S reply:                by SSR fruits,        by synchronous mode S replies,        by multipaths of the mode S reply.        
The correction principle described hereinabove does not withstand certain extreme configurations encountered on different radar sites, three exemplary (but nonlimiting) cases of which can be taken:                in the case of FIG. 3, encountered in Northern Europe: in an environment polluted by numerous asynchronous secondary replies (called “fruits”), it is probable that from one mode S selective interrogation to the next the associated replies will be polluted each time by an SSR-fruit reply of significantly higher power than the mode S reply (case of a wanted target at a great distance of 470 km and of fruit generated by targets close to the radar concerned, but replying to a distant radar). The fruits, being asynchronous, provoke errors on different bits from one mode S reply to the next. FIG. 3 shows, at the start of the time line, the four unpolluted mode S reply preamble pulses. Then come the data bits (referenced bit 1 to bit 56 in the figure), the first of which are polluted by spurious pulses from a full-code SSR reply (shown shaded in the figure, and with an amplitude greater than that of the wanted bits) arriving asynchronously in relation to the wanted pulses. Such spurious pulses are just as likely to back-fill the intersymbol spaces as they are to overlap more or less significantly the forward pulses.        A second case is illustrated by FIG. 4. The pollution is due to synchronous spurious replies originating from different aircraft for interrogations from one and the same radar. In the mode S reply acquisition phase (“All Call”) in an environment filled with targets such as a Northern European air corridor, the wanted mode S replies are mutually polluted synchronously. The false bit rate depends on the mutual overlap rate of the mode S replies. From recurrence to recurrence, the errored bits may not always be the same because of beats between the signals of different frequencies. Thus, in the case of FIG. 4, while a radar begins to receive a mode S reply from a first aircraft, a reply originating from a second aircraft begins to arrive from the second data bit of the first reply. The four synchronization pulses are such that the first of them is placed between the second and the third pulses of the first reply, whereas the other three overlap the bit 3 to bit 7 pulses of the first reply in different ways, because the respective distances of the synchronization pulses are not the same as those of the data pulses. Then, the data pulses of the second reply overlap the data pulses of the first.        FIG. 5 relates to the case of a multipath propagation of one and the same reply. In the presence of strong multipaths, when the TS decodes bits badly, the bits can be distributed anywhere in the reply since, by nature, the multipath can pollute all the bits of the message. In practice, since the multipaths are the same reply repeated and offset in time by a duration that can be as much as 3 μs, the badly decoded bits depend on the message itself and on the beats of the signals (direct reply and reply from multipaths) in the receiver, which distorts the pulses at the output of the receiver. Consequently, the TS which exploits the received power, may wrongly position the pulses, wrongly assign a power to the latter and consequently wrongly decode the reply. Now, the principle of the error detecting code cannot be used to correct errors spaced apart by more than 24 μs. From one mode S reply to the next, the errored bits are not the same, because the distortion of the pulses due to the beating between the forward wave and the reflected waves depends on the tread difference which changes sufficiently from recurrence to recurrence (10 ms).        
The market, in the new uses of the mode S radars, increases the need to detect a target on the basis of few mode S interrogations, even over and above the need to have a good effectiveness indicator since:                the speed of rotation of the antenna of the surveillance radar is increased: often one revolution in four seconds for a range of 470 km. Consequently, the illumination time on a target is reduced and, because of this, the possibility of reinterrogating in case of failure is more limited,        the mode S data transactions require illumination time on the target, so reducing the possible number of recurrences for a reinterrogation in the event of failure on the previous attempt. Military radars require additional interrogations in specific military modes (1 and 2), so further reducing the number of recurrences for the mode S.        
The processing of replies from secondary radars performed by the Applicant since the 1990s has undergone two main developments, which are illustrated in the block diagrams of FIG. 6 (at the top and in the middle, respectively), as is the inventive solution (diagrammatically represented at the bottom of the figure). The secondary radars that implement these three different extraction techniques all have three main stages, corresponding to three main steps of the blip extraction process and represented in the same columns of the drawing: a radiofrequency processing stage 1, a signal processing stage 2 (hereinafter called SP) and a data processing stage 3 (hereinafter called DP). The stage 1 is the same for all three implementations. It essentially comprises a radar antenna 4, a receiver 5 and an interrogator 6. For stage 2, a number of successive quality detection and determination processing operations are diagrammatically represented, at the output of the receiver of each of the three methods, one under the other, corresponding to successive interrogations.
The two known techniques are:
1. “Reply Processing and Correlator R.P.C.”. This is a secondary extractor developed between 1992 and 1999 and for which a number of patents have been filed, relating only to the innovative SSR (Secondary Surveillance Radar) processing, characterized by a strong discrimination capability based on the analysis of the form of the signals received on the Σ channel. The secondary processing principle is based on the systematic interrogation of all the targets present in the lobe at a rate of a dozen replies (6 in mode A and 6 in mode C) per target in the lobe. The main functionalities are managed as follows (see FIG. 6):                Space-time management (GST): this is managed by the elements 7 (pacing of the beam) and 8 (GST) and it is very simple since the sequencing systematically comprises interleaved mode A and mode C interrogations.        Signal processing (TS):                    This detects and decodes the SSR replies on the basis of the analysis of the form of the signals received on the Σ channel,            It establishes a quality constructed on the basis of the analysis of the Σ and Δ/Σ information.                        Data processing—TD—(9) handles the extraction of the blip on the basis:                    of the number of detections for each mode or for all modes to detect the blip,            of the generation of the mode A/mode C codes by analysis of the codes obtained in each mode associated with their qualities and based on an estimator for each pulse of the code using the flags (flags giving the risks of garbling of a reply, therefore its potential to be correctly decoded).                        
The blocks of the diagram in the drawing show the degree of complexity of the various main functions reviewed hereinabove:                Space-time management (GST): low complexity,        Signal processing (TS): average complexity,        Data processing (TD): average complexity.        
2. “Interrogator and Reply Processing” “I.R.P.”. This is a secondary extractor developed between 1999 and 2005. A number of patents have been filed for the innovative mode S signal processing aiming for a strong discrimination capability based on histograms of the pulses defined by the analysis of the form of the signals received on Σ and on Δ, and another has been filed for the innovative mode S data processing in the sequencing of the mode S selective interrogations. The principle of the mode S processing is based on the selective interrogation of each target in the lobe at a rate of two replies per target in the lobe:                Space-time management (GST): this is managed by the elements 10 (pacing of the beam in mode S) and 11 (GST in mode S): this is highly sophisticated, since the sequencing is conditioned both by the principal sequencing selected by the operator within which should fall in real time all the selective interrogations and the placement of the listening windows associated with the replies expected from a chosen target (50 targets per lobe),        Signal processing (TS): this is highly sophisticated:                    It detects the mode S pulses on the basis of the analysis of the form of the signals received on the Σ and Δ channels and pulse histogram,            It establishes a constructed quality of each pulse on the basis of histograms of the Σ, Δ and Δ/Σ pulses,            It handles the detection of the reply on the basis of the detected pulses,            It handles the decoding of the reply message on the basis of the detected pulses and of the associated qualities for each bit of the message,            It performs (in 12), independently for each reply, the calculation of the message error syndrome, and, if necessary, it tries to correct the message on the basis of the quality associated with each pulse.                        Data processing (TD) simply handles (in 13) the association of the replies for a target that has already been isolated by the SP and the calculation of its general characteristics (power, azimuth, distance).        
The blocks of the diagram in the drawing show the degree of complexity of the various main functions reviewed hereinabove:                Space-time management (GST): highly complex,        Signal processing (TS): highly complex,        Data processing (TD): low complexity.        
Currently, the SP determines, for each detected reply, a reference value according to the three conventional variables (in SUM, in DIFFERENCE and in “MONOPULSE”) and the maximum number of samples that are consistent with respect to this value for the three said variables, these samples hereinafter being called “consistent samples”. This also indicates the overall quality of the reply: the higher this maximum number of consistent samples becomes, the clearer the overall quality becomes (unpolluted)
The decoding of each bit, and the quality (uncertainty as to its value), is established in relation to the position of the pulse or of the pulses in the period of the bit and of the value according to the three said variables of the pulse or pulses in relation to the value of the reply for these three variables.
The block diagram of FIG. 7 is, by way of example, a detail view of a few bits of the message. It shows the difficulty in decoding certain bits when the message is polluted by a number of multipaths:
the first line is a simplified representation of the signals received at the input of the receiver:                the wanted signal of the reply        a slightly weaker multipath offset by 500 ns        a second weaker multipath offset by 800 ns        
the second line shows, for the SUM pathway or the DIFFERENCE pathway, the signal at the output of the receiver that exploits the TS to define the presence of a pulse and its value. The broken line depicts the power of the reply calculated over all the pulses in a position to belong to the reply.
the bottom part of the figure gives a possible result of the TS relating to the establishment of the values of the pulse.